A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 2 - Lakes

Nutrient Concentrations in Lakes

Why Are They Important?

Nutrients is lakes serve the same basic functions as nutrients in a garden. They are essential for growth. In a garden, growth and productivity are considered beneficial, but this is not necessarily so in a lake. The additional algae and other plant growth allowed by the nutrients may be beneficial up to a point, but may easily become a nuisance.

The main nutrients of concern are phosphorus and nitrogen. Both elements are measured in several forms. Phosphorus can be measured as total phosphorus (TP) or as soluble reactive phosphate (SRP). SRP is also sometimes called phosphate (PO4) or orthophosphate (ortho-P). SRP represents the fraction of TP that is available to organisms for growth.

Nitrogen can be measured as total nitrogen (TN), total Kjeldahl nitrogen (TKN), nitrate-nitrogen (NO3), nitrite-nitrogen (NO2) [these are usually measured as nitrate-nitrite-nitrogen (NO3 – NO2), or ammonia-nitrogen (NH4). TN is similar to TP and is used to represent the total amount of nitrogen in a sample. TKN represents the fraction of TN that is unavailable for growth or bound up in organic form; it also includes Nh . The remaining fractions, NO3 – NO2 and NH4 , represent bioavailable forms of nitrogen. If they are summed, they can be compared to the SRP fraction of phosphorus.

One chemical form of an element can be converted into another. The conditions under which the conversion occurs are influenced by many factors, such as pH, temperature, oxygen concentration, and biological activity.

The total concentration of a nutrient (e.g., TP or TN) is not necessarily the most useful measurement. For example, if a sample is analyzed for TP, all forms of the element are measured, including the phosphorus "locked up" in biological tissue and insoluble mineral particles. It may be more useful to know the concentration of phosphorus that is actually available for growth. SRP better reflects bioavailability.

Although there are many different forms of nutrients that can’t be measured, there are only three commonly used combinations. These are (1) measure all forms of both elements – TP, SRP, TN, NO3- NO2, NH4; (2) measure only total nutrients – TP and TN; or (3) measure only available nutrients – SRP, NO3- NO2, and   NH4. (In the first example, TKN could be measured instead of TN. Depending upon which form is measured, the other can be estimated by difference.)

Reasons for Natural Variation

The concentration of nutrients and the forms they are found in change continually. How and why they change is a very complex field of study. The total input of nutrients varies through time, depending upon land use and other factors. During the summer, nutrient input may increase due to fertilization of cropland, lawns, and gardens. During the winter, high rainfall causes increased washoff of organic mater such as leaves, twigs, grass, and other debris. Because decomposition of this organic matter releases nutrients, it constitutes an important source of nutrient loading.

Whether the increase in total nutrient concentrations results in higher available nutrient concentrations, and therefore an immediate increase in growth or productivity, depends upon the original form of the nutrient and physical conditions. If nutrients enter as organic matter that first needs to be decomposed before if can be utilized for growth, temperature becomes important because of its effect on the rate of decomposition. (During warmer months, nutrients entering the system as intact organic matter would be decomposed relatively quickly as compared with cold, wet-weather months when decomposition is slow.)

These dynamics are further complicated by the fact that increased growth leads to greater numbers of organisms, which need even more nutrients. So, as nutrients become available they are immediately utilized. In this case, an increase in total nutrients would not be reflected by any measurable increase in available nutrient fractions. In short, clear or simple relationships between increases in organic matter or other sources of nutrients and resultant increases in either total or available nutrient concentrations become obscure.

Nutrient concentrations also may vary with depth in a lake. Near the top of the lake, where light stimulates algae growth, total nutrient concentrations may be higher than those deeper in the lake. These high total concentrations reflect the increased concentration of organic matter – algae. But because the organisms are utilizing most of the nutrients that are produced, available nutrient concentrations may be low. Since decomposition of organic matter – formation of available nutrients from total nutrients – occurs to a larger extent near the bottom of a lake, available nutrient concentrations may be higher at depth.

Expected Impact of Pollution

Most sources of pollution to lakes contribute nutrients in one form or another. These sources include stormwater runoff, which may carry fertilizers from lawns and cropland as well as organic matter such as leaves, grass, and insects; waste products from farm animals and domestic pets; failing lakeside septic systems; and effluent from industrial and municipal wastewater treatment plants. As the number or size of pollutant sources increases, average nutrient concentrations also increase.

Nutrient concentrations are reported in units of milligrams of nutrient per liter of water – mg/L, or micrograms per liter of water ug/L. Milligrams per liter is equivalent to parts per million (ppm); micrograms per liter is equivalent to parts per billion (ppb). Nutrient concentrations for three Western Washington lakes are shown below to provide examples of the range you may expect to measure.

Total phosphorus concentrations can be used to determine a lake’s trophic status. Though trophic status is not related to any water quality standard, it is a mechanism for "rating" a lake’s productive state. Information on calculating trophic status is included in the interpretation section at the end of this chapter.

The next section discusses total suspended solids and turbidity in lakes.

Chapter Four provides information about how to measure nutrients in lakes.

Temperature | Oxygen | pH | Secchi | Nutrients | Turbidity | Chlorophyll | Fecal Coliforms

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